Another important factor affecting SHG conversion efficiency is phase matching between the fundamental and second harmonic waves: chromatic dispersion of the material typically leads to dephasing and thus a periodic exchange of energy between fundamental and harmonic wave instead of a continuous increase of second harmonic energy along the propagation direction. Among the different techniques that can be used to achieve phase matching, the most popular approaches are to use either a birefringent nonlinear crystal or to employ quasi-phase-matching (QPM) [1]}. This last technique works by periodically modulating the sign of the \(\chi ^{(2)}\)  susceptibility to compensate for the phase mismatch acquired between fundamental and harmonic wave during propagation.
Phase matching can also be achieved by tailoring the dispersion of a linear grating [2]}, [3]}, [4]}.
